Position detector

Abstract: A position detector which provides improved reference position detecting precision by stabilizing the output signal from magnetoelectric transducers in a reference position part. A rotor (2) has a magnetized surface (5) alternately provided with different magnetic poles and a reference positional part (5a) in which magnetic poles provided on the magnetized surface (5) are partly omitted. A rotational angle sensor (6) has MR elements (7A and 7B) laid out thereon in opposition to the magnetized surface (5) of the rotor (2) to output the voltage signal VS according to the magnetic field intensity in the magnetized surface (5) due to the rotation of the rotor (2). A waveform processing circuit (11) and a pulse signal processing signal (12) shape the waveform of the voltage signal (VS) from the rotational angle sensor (6), and also generate the rotational angle pulse signal (Ne) and a reference position pulse signal (G) according to the waveform reforming signal. The reference positional part (5a) is arranged so that the magnetic field intensity therein can be within the saturation magnetic field region of the Mr elements (7A and 7B).

Description: BACKGROUND OF THE INVENTION Field of the Invention: The present invention generally relates to a moving position detector. More particularly, the present invention relates to a rotational position detector which utilizes magnetoelectric transducers having the saturation characteristics as to voltage output, such as magnetoelectric transducer elements composed of a ferromagnetic material, e.g., Ni-Co type alloy and Ni-Fe type alloy (hereinafter referred to as "MR elements"), and nonlinear type Hall elements. Description of the Prior Art: A rotational position detector in which the outer periphery of a rotor has a magnetized surface alternately provided with different magnetic poles and the magnetized surface is detected by using a rotational angle sensor utilizing magnetoelectric transducers has been proposed as disclosed, for example, in Japanese Unexamined Patent Publication No. 54-156656, Japanese Unexamined Utility Model Publication No. 56-12815 or Japanese Examined Patent Publication No. 2-52966. The rotational angle sensor of the above rotational position detector comprises a single or a plurality of magnetoelectric transducers to obtain the output voltage from the rotational angle sensor by means of the magnetoelectric transducing action according to the magnetic field intensity. On the other hand, the magnetized surface of the rotor is alternately provided with different magnetic poles and a reference position part magnetized with a larger magnetization pitch is formed in part of the magnetized surface to have a single unit of rotor and rotational angle sensor generates rotational angle signals and reference position signals. Furthermore, the waveform of the output signal from the rotational angle sensor is shaped, and the rotational angle signal is generated according to the reference position signal. During this process, the output voltage from the magnetoelectric transducer becomes constant (e.g., at "0" value) in the reference position part for the specified duration, and consequently the reference position signal can be obtained. However, in the rotational position detector in the above arrangement, when the magnetization pitch of magnetic poles is widened as shown in (A) of Fig. 17, the magnetic field intensity in the reference position part lowers at the central part thereof as shown in (B) of Fig. 17. As a result, a swell or fluctuating noise as shown in (C) of Fig. 17 is caused on the output voltage signal from the rotational angle sensor in the reference position part according to the variation in the magnetic field intensity. In addition to the above problem, when ferromagnetic MR elements composed of a ferromagnetic material, such as a Ni-Co type alloy, are used as magnetoelectric transducers, there are the adverse effects of the hysteresis characteristics as to the output voltage Vm in; the range A of magnetic field intensity Hy as shown in Fig. 16. Moreover, the hysteresis characteristics themselves vary according to the material manufacturing conditions, i.e, the hysteresis characteristics differ from product to product. This makes the output waveform in the reference position part more unstable. For these reasons, the reference position detecting precision is lowered. Therefore, when the conventional rotational position detector is used for operational environment or rotation which is subject to a large variation or to high-precision high-volume pulses, such as rotational position detectors for automobile engines, these problems may severely restrict particularly on designing rotational angle sensors. Furthermore, document EP 0 412 017 discloses a detection device, wherein a rigid ring includes multiple north and south magnetic poles destributed alternately over the entire circumference of the ring. The magnetic poles are arranged to have a certain number of poles of a smaller extent and, in dependence of detecting conditions, a certain number of poles of a larger extent. However, a variation in the magnetic field intensity of the magnetic poles of the larger extent in comparison with a predetermined field intensity is not taken into consideration. Document US 4 866 381 discloses a plural magnet device for magnetically detecting rotation angle of a rotary body, wherein multiple north and south magnetic poles are distributed alternately on the circumferential surface of the rotary body. The magnetic poles arranged on the circumferential surface are provided with a different width so that the portions of the circumferential surface have a width which is different from that of a plurality of magnets on the remaining portions. The detection of the rotational angle of the rotary body is carried out by a single rotation angle sensor arranged to oppose the circumferential surface of the rotary body. The rotation angle sensor outputs an electric signal indicating the scanning process of the magnets when the rotary body is rotated. Due to the different size of the adjacent magnets, flux density changes at locations, where there is a change in magnet width. The rotational position is then determined by evaluating the output signal of the rotation angle sensor. Moreover, document US 4 987 415 discloses a high resolution encoder having a relatively high number of magnetic pole pairs with a magnetic flux strength sufficient for use in high resolution applications. The high number of magnetic pole pairs ensures that a Hall sensor installed in the proximity of the encoder will switch on and off through an air gap which has some maximum value for a specific product configuration. The high number of magnetic pole pairs ensures a high resolution of the encoder. SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to solve the above problems by providing a position detector which provides the improved reference position detecting precision by stabilizing the output signal from magnetoelectric transducers in the referenced positional part. In order to achieve the above object, the present invention provides a position detector comprising a moveable body having a surface magnetized in opposite polarities alternately at a first predetermined magnetization pitch on a surface thereof except at a reference position part which has a second magnetization pitch larger than said first magnetization pitch, and a magnetoelectric transducer positioned to face said moveable body for generating a voltage signal corresponding to an intensity of said magnetized surface, said transducer element having an output saturation characteristics in a region where said magnetic field intensity is above a predetermined intensity, characterized in that said magnetic field intensity caused by said reference position part in said magnetoelectric transducer is above said predetermined intensity, when the reference position part is opposite said magnetoelectric transducer. Accordingly, in the first or second embodiment of the present invention, the magnetism of a pair of magnetic poles provided in opposition to each other in the reference positional part of the rotor may be set stronger than that of the other magnetic poles. Moreover, in the first or second embodiment of the present invention, the magnetization width of a pair of magnetic poles provided in opposition to each other in the reference positional part of the rotor may be set wider than that of the other magnetic poles. In accordance with the first embodiment of the invention, the magnetoelectric transducers output the voltage signal according to the magnetic field intensity of the magnetized surface of the rotor. At this time, as the magnetic field intensity in the reference positional part of the rotor is within the magnetic field region in which the output voltage from the magnetoelectric transducers saturates, the voltage signal can hold the adverse effects of the magnetic field decrease in the reference position part extremely small. As a result, the output voltage from the magnetoelectric transducers corresponding to the reference positional part can be held at a constant level and the generation of swell or fluctuating noise can be controlled. In accordance with the second embodiment of the invention, the ferromagnetic reluctance elements as magnetoelectric transducers output the voltage signal according to the magnetic field intensity of the magnetized surface of the rotor. At this time, as the magnetic field intensity in the reference position part is within the reversible magnetic field region of the ferromagnetic reluctance elements, the output from the ferromagnetic reluctance elements in the reference position part can prevent the adverse effects of the hysteresis. As a result, the reference position detecting precision can be improved. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Fig. 1 is a schematic view showing a rotational position detector according to the first embodiment of the present invention; Fig. 2 is a front view illustrating the shape of MR element according to the first embodiment; Fig. 3 is an electric block diagram illustrating the configuration of a waveform processing circuit and a pulse signal processing circuit according to the first embodiment; Fig. 4 is a time chart illustrating magnetization pattern (A), magnetic field intensity (B), and waveforms (C) through (G) of the rotational position detector according to the first embodiment; Fig. 5 is a characteristic diagram illustrating the relation between the magnetic field intensity and output voltage from the MR element according to the first embodiment; Fig. 6 is a waveform chart illustrating the magnetization pattern (A) of the magnetized surface and the magnetic field intensity (B) according to the second embodiment; Fig. 7 is a waveform chart illustrating the magnetization pattern (A) of the magnetized surface and the magnetic field intensity (B) according to the third embodiment; Fig. 8 is a partial plan view illustrating a magnet according to the fourth embodiment; Fig. 9 is a partial plan view illustrating a magnet according to the fifth embodiment; Fig. 10 is a partial plan view illustrating a magnet according to the sixth embodiment; Fig. 11 is a partial plan view illustrating a magnet according to the seventh embodiment; Fig. 12 is a partial plan view illustrating a magnet according to the eighth embodiment; Fig. 13 is a partial plan view illustrating a magnet according to the ninth embodiment; Fig. 14 is a partial plan view illustrating a magnet according to the tenth embodiment; Fig. 15 is a partial plan view illustrating a magnet according to the eleventh embodiment; Fig. 16 is a characteristic diagram illustrating the relation between the magnetic field intensity Hy and output voltage Vm from the conventional MR elements; and Fig. 17 is a waveform chart illustrating the magnetization pattern (A), magnetic field intensity (Hy) and sensor output (C) for use in explaining a prior art. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) This invention may best be understood with reference to the following description of the presently preferred first embodiment materialized in a rotational position detector for use in a four-cycle four-cylinder engine together with the accompanying drawings. As illustrated in Fig. 1, a disk-shaped rotor 2 is fixedly installed on a shaft 1 rotating at the ratio of 1/2 rotation to one rotation of the engine. The rotor 2 comprises a magnet holding part 3 composed of a non-magnetic material (e.g., austenitic stainless steel) and an annular isotropically sintered ferrite magnet 4 (hereinafter referred to as "magnet") adhered to the outer periphery of the magnet holding part 3. In this embodiment, the residual magnetic flux density Br of the magnet 4 is set to 0,2 T to 0,25 T (2000 to 2500G). As the specific dimensional values of the magnet 4 of this embodiment, the diameter ø is 80mm, the thickness Wm in the axial direction is 8mm and the thickness t in the radial direction is 4mm. The outer periphery of the magnet 4 constitutes a magnetized (circumferential) surface 5 alternately provided with magnetic poles (N and S poles) at regular intervals (i.e., magnetization pitch λ ). The magnetized surface 5 includes a reference position part 5a with the wider magnetization pitch λ0 in four positions at every 90°, each being correspondent to each cylinder of a four-cylinder engine (not shown). The relation between the magnetization pitch λ0 of the reference position part 5a and the magnetization pitch λ of the other magnetic poles N,S is set to be λ0 is equal to 3·λ, and in this embodiment, the magnetization pitch λ is set to 5°. A rotational angle sensor 6 has a pair of MR elements 7A and 7B as magnetoelectric transducers, which are laid out thereon in opposition to the magnetized surface 5 of the rotor 2. The distance tg between the MR elements 7A and 7B and the magnetized surface 5 should be determined taking the vibration of an applied car, the dimensional tolerances of applied parts and components, the assembly tolerance thereof, etc. into account, and in this embodiment, the distance tg is set to 1.5mm. The MR elements 7A and 7B are composed of a ferromagnetic material, such as Ni-Cd type alloy. The installation pitch λ1 of the MR elements 7A and 7B is approximately 1/2 of the magnetization pitch λ(i.e., λ1 is equal to λ/2). A power source (with the power voltage of 5V) 8 is serially connected to the MR elements 7A and 7B. Fig. 2 illustrates the shape pattern of the MR element 7A or 7B. In this embodiment, the MR elements 7A or 7B are triply folded, and a terminal 9 is connected to each end thereof. As the specific dimensional values of this fold, the length L is 3mm and the width We is 30µm. Furthermore, a waveform processing circuit 11 is connected to an intermediate point 10 between the MR elements 7A and 7B. The MR elements 7A and 7B vary the electric resistance values thereof according to a magnetic field intensity Hy in the circumferential direction with a delay due to the rotation of the rotor 2 by the installation pitch λ1 (which is equal to λ/2). As a result, a voltage of the intermediate point 10 between the MR elements 7A and 7B varies, and a voltage signal Vs is output from the rotational angle sensor 6 to the waveform processing circuit 11. The waveform processing circuit 11 shapes the waveform of the voltage signal Vs from the rotational angle sensor 6. A pulse signal processing circuit 12 is connected to the waveform processing circuit 11 to generate a rotational angle pulse signal Ne and the reference position part pulse signal G according to the waveform shaped signal. Fig. 3 is a block diagram illustrating the configuration of the waveform processing circuit 11 and pulse signal processing circuit 12. As shown in this figure, an AC coupling 13 of the waveform processing circuit 11 is connected to the output terminal of the rotational angle sensor 6. The AC coupling 13 removes only the DC component from the voltage signal Vs output from the rotational angle sensor 6 and extracts only the AC component therefrom. To the AC coupling 13 is connected an amplifier 14. To the output side of the amplifier 14 is connected the input side of a comparator 17. With the comparator 17 are connected the output side of a threshold level generating circuit 15 through a threshold level selecting circuit 16. The threshold level generating circuit 15 outputs the first threshold level Vthl and the second threshold level Vth2, while the threshold level selecting circuit 16 selectively outputs either the first threshold level Vthl or the second threshold level Vth2 according to the output of the comparator 17. The comparator 17 generates a square wave according to the output signal from the amplifier 14 and the first threshold level Vthl or the second threshold level Vth2 from the threshold level selecting circuit 16. Incidentally, the threshold level generating circuit 15 sets the values of the first threshold level Vthl and second threshold level Vth2 so that the duty ratio of the waveform of the output signal from the comparator 17 can be 50%. On the other hand, a rise and fall edge detecting circuit (hereinafter referred to as "edge detecting circuit") 18 for the pulse signal processing circuit 12 is connected to the comparator 17. The edge detecting circuit 18 detects the rise and fall edges of the square wave signal output from the comparator 17 and then outputs the pulse signal in correspondence to each edge. In this way, the edge detecting circuit 18 outputs the rotational angle pulse signal Ne. The edge detecting circuit 18 and a clock generating circuit 19 are connected to the input side of a period computing circuit 20. The period computing circuit 20 computes the period of the rotational angle pulse signal Ne output from the edge detecting circuit 18 according to the number of clocks from the clock generating circuit 19. To the period operating circuit 20 is connected a reference signal generating circuit 21 comprising a counter part 22 and a period judging part 23. It is so arranged that the output signal from the period computing circuit 20 is input into the clock terminal of the counter part 22, while the output signal (or the rotational angle pulse signal Ne) from the edge detecting circuit 18 is input into the reset terminal of the counter part 22. The counter part 22 performs count-up and count-down operations according to the clock and reset signals. Also to the counter part 22 is connected the period judging part 23 operating a period ratio of Tn/Tn-1 according to the output period T of the rotational angle pulse signal Ne, where the subscript "n" is the current value and the subscript "n-1" is the previous value. The period judging part 23 outputs the reference position part pulse signal G according to a period ratio of Tn/Tn-1. Now, the variation in the resistance value of the MR elements 7A and 7B in correspondence to the magnetic field intensity Hy will be described. Fig. 5 is a Hy-Hv characteristic diagram illustrating the variation in the resistance value of the MR elements 7A and 7B by using the output voltage Vm. In the characteristic diagram illustrated in Fig. 5, the output voltage Vm from the MR elements 7A and 7B have hysteresis characteristics within the region between reversible magnetic field intensities Hr and -Hr which are the hysteresis generation limits. In the regions outside the hysteresis region between -Hr and Hr exist saturated magnetic field intensities Hsat and -Hsat outside which saturate the output voltage Vm from the MR elements 7A and 7B at a saturation voltage V0. In this embodiment specifically arranged as described in the above, the saturated magnetic field intensity Hsat of the MR elements 7A and 7B is set to approximately 8·10 3 A/M (100Oe) and the reversible magnetic field intensity Hr thereof is set to approximately 5,6·10 3 A/M (70Oe). On the other hand, the magnetic field intensity Hy in the reference position part 5a of the magnetized surface 5 is set so as to be maintained within the saturated magnetic field region (Hy is larger or equal to 8·10 3 A/M (100) or Hy is smaller or equal to -8·10 3 A/M (-100)). Therefore, the output voltage Vm from the MR elements 7A and 7B can be maintained at the saturation voltage V0 or more in the reference position part 5a. Next, the operation of the rotational position detector according to the first embodiment of the present invention will be described. Fig. 4 is a time chart illustrating the magnetic field intensity Hy of the magnetized surface 5 and the operation of the rotational position detector in correspondence thereto. As shown in this chart, the magnetic field intensity Hy in the circumferential direction shown in (B) of Fig. 4 varies according to the magnetized pattern of the magnetized surface 5 shown in (A) of Fig. 4. The rotational angle sensor 6 outputs the voltage signal Vs, as shown in (C) of Fig. 4, at the ratio of 2 cycles to one cycle of variation in the magnetic field intensity Hy due to the rotation of the rotor 2. As aforementioned, as Hy is larger than or equal to Hsat in the reference position part 5a of the magnetized surface 5, both the MR elements 7A and 7B operate within the saturated magnetic region. Accordingly, the voltage signal Vs from the rotational angle sensor 6, which is the intermediate point voltage from the MR elements 7A and 7B, can substantially reduce the swell or fluctuating noise of the output voltage caused by the reduction in the magnetic field intensity at the central part of the reference position part 5a. Therefore, the voltage signal Vs can be maintained at a swell noise level of approximately "0" value in the reference position part 5a. In the waveform processing circuit 11 illustrated in Fig. 3, the comparator 17 generates as shown in (D) of Fig. 4 signal waveform with a duty ratio of 50% according to the voltage signal Vs, which is input from the rotational angle sensor 6 through the AC coupling 13 and the amplifier 14, and first threshold level Vthl or the second threshold level Vth2, which is set by the threshold level secreting circuit 16. That is, the comparator 17 outputs the H (high) level signal within the shifting range from the first threshold level Vthl to the second threshold level Vth2, and then outputs the L (low) level signal within the shifting range from the second threshold level Vth2 to the first threshold level Vthl. As the voltage signal Vs from the rotational sensor 6 is maintained at approximately "0" value in the reference position part 5a, the output signal of the comparator 17 is also maintained at the L level. As a result, the output signal of the waveform processing circuit 11 is a square wave with a duty ratio of 50% in any part other than the reference position part 5a, and the waveform thereof has an omission only a the part which is in correspondence to the reference position part 5a. In the pulse signal processing circuit 12, the edge detecting circuit 18 generates as shown in (E) of Fig. 4 the rotational angle pulse signal Ne for each specified rotation angle by using the rise and fall edges of the output signal from the waveform processing circuit 11. The rotational angle pulse signal Ne output from the edge detecting circuit 18 has a signal waveform with an omission only in the' part corresponding to the reference position part 5a like the above output waveform from the waveform processing circuit 11. The counter part 22 of the reference signal generating circuit 21 performs the count-up and count-down operations according to the number of clocks from the period computing circuit 20. To be specific, as shown in (F) of Fig. 4, the counter part 22 switches to the count-down operation when the reset signal (the output signal from the edge detecting circuit 18) is input thereinto during the count-up operation, and resumes the count-up operation after once resetting the output signal when the reset signal is input thereinto during the count-down operation. In the reference position part 5a, however, as the rotational pulse signal Ne is omitted, the reset signal is not input into the counter part 22 during the count-down operation. As a result, the counter part 22 continues the count-down operation until the count value becomes "0." The period judging part 23 judges whether a period ratio of Tn/Tn-1 during the continuous count-up and count-down operations is at the specified preset value m or larger (m is 3 in this embodiment) or not. As shown in (G) of Fig. 4, when Tn/Tn-1 is smaller than 3, the period judging part 23 maintains the output signal level at the L level, and when Tn/Tn-1 is larger than/equal to 3, it maintains the output signal level at the H level. That is, as the rotational angle pulse signal Ne is output for each specified angle in any part other than the reference position part 5a, Tn is equal to Tn-1 and the output signal level of the period judging part 23 is maintained at the L level. In the reference position part 5a, however, as the rotational pulse signal Ne is omitted, Tn/Tn-1 is larger than/equal to 3 and the output signal level of the period judging part 23 is maintained at the H level. Then, when the reset signal is output again from the edge detecting circuit 18 to the counter part 22, the counter part 22 resumes the count-up operation. As Tn/Tn-1 becomes smaller than 3 again, the period judging part 23 reduces the output signal level to the L level. In this way, as the rotor 2 rotates, the rotational angle pulse signal Ne is output for each specified angle from the rotational angle sensor 6, and one reference position part pulse signal G at the H level is output in correspondence to the reference position part 5a. As described in detail in the above, the rotational position detector according to this embodiment is arranged so that the rotational angle sensor 6 is composed of the MR elements (ferromagnetic reluctance elements) 7A and 7B. The MR elements 7A and 7B are laid out in opposition to the magnetized surface 5 of the rotor 2, which is alternately provided with different magnetic poles. The magnetized surface 5 of the rotor 2 includes the reference position part 5a provided with magnetic poles at a wider magnetization pitch, and the magnetic field intensity Hy in the reference position part 5a is within the saturated magnetic field region of the MR elements 7A and 7B. In this arrangement, the output voltage Vm corresponding to the resistance value from the MR elements 7A and 7B in correspondence to the reference position part 5a is not affected by the hysteresis, and moreover, even if there is a swell in the magnetic field intensity Hy, it is maintained at an almost constant saturation value. Accordingly, the voltage signal Vs from the rotational angle sensor 6 is maintained at an almost constant value. As a result, the swell noise of the rotational angle sensor 6 can be controlled, the precision of detecting the reference position part 5a can be improved, and the S/N ratio can be set always to a high value. In the above embodiment, the magnetic field intensity Hy in the reference position part 5a of the rotor 2 is set to be higher than the saturated magnetic field intensity Hsat. The magnetic field intensity Hy, however, can be set to the intensity in the magnetic field region between the reversible magnetic field intensity Hr and the saturated magnetic field intensity Hsat (within 5,6·10 3 to 8·10 3 A/M (70 to 100Oe)). In this case, as the output voltage (resistance value) from the MR elements 7A and 7B in the reference position part 5a varies without being affected by the hysteresis, the output voltage from the rotational angle sensor 6 in the reference position part 5a can be constantly obtained. As a result, the same advantages of the invention can be obtained as those of the above embodiment. Furthermore, the specific values are set in the above embodiment to obtain the desired saturated magnetic field intensity Hsat and reversible magnetic field intensity Hr. These value, however, should not be set only to the above specific values but may be arbitrarily set to any value according to the operational condition and required precision. Moreover, although the rotational angle sensor 6 comprises a pair of half bridges in the previous embodiment, it can use a full bridge instead. It is also possible that a more than one pair of sensors are laid out on a single board at regular intervals to double or triple the number of pulses. (Second Embodiment) The second embodiment of the present inventing will be described referring only to the differences from the first embodiment. Fig. 6 illustrates in (A) and (B) the magnetized pattern of the magnetized surface 5 and the magnetic field intensity Hy corresponding to the magnetized pattern according to the second embodiment, respectively. As shown in this figure, the magnetized surface 5 is alternately provided with different magnetic poles at the magnetization pitch λ0 on one hand, and the magnetized surface 5 included the reference position part 5a provided with magnetic poles at the magnetization pitch λ0 (λ0 = n·λ, where n = 2 to 4) on the other hand. Furthermore, the both sides of the reference position part 5a are provided with the magnetic poles with higher intensity than that of the other magnetic poles as shown by N+ and S+ in (A) of Fig. 6. In this arrangement, the magnetic field intensity Hy in the reference position part 5a is higher, as shown by a solid line in (B) of Fig. 6 compared with a case where magnetic poles with a normal level of magnetism are provided as shown by a broken line in (B) of Fig. 6. As a result, the magnetic field intensity Hy in the reference position part 5a is maintained within the saturated magnetic field region. As the MR elements 7A and 7B become to operate within the saturated magnetic field region accordingly, the swell noise of the output voltage VS from the rotational angle sensor 6 in the reference position part 5a can be stabilized at a level of approximately "0" value. In the second embodiment, as understood in the above description, the MR elements 7A and 7B are arranged for operation within the saturated magnetic field region by strengthening the magnetism of the magnetic poles in the reference position part 5a. As a result, also in the second embodiment, the adverse effects of the hysteresis on the output voltage (or resistance value) from the MR elements 7A and 7B can be prevented, and the same effects and advantages as those of the first embodiment can be obtained. (Third embodiment) Next, the third embodiment of the present invention will be described. Fig. 7 illustrates in (A) and (B) the magnetized pattern of the magnetized surface 5 and the magnetic field intensity Hy corresponding to the magnetized pattern according to the third embodiment, respectively. the rotational position detector according to the third embodiment is different from the first and second embodiments in that a radial anisotropic ferrite type plastic magnet is used. As shown in (A) of Fig. 7, the magnetized surface 5 of the magnet 4 is alternately provided with magnetic poles with different directions in such a way that the S and N poles are laid out n opposition to each other both in the vertical and circumferential directions. On the other hand, part of the magnetized surface 5 includes the reference position part 5a which comprises non-magnetized part (where weakly magnetized part can also be used) with the magnetization width Wp". The magnetic poles of the magnetized surface 5 are provided with the magnetization width Wp, while only the magnetic poles in opposition to each other in the reference position part 5a are provide with the magnetization width Wp' where Wp' is larger than Wp. In this embodiment, the magnetization widths are set to be Wp' = n·Wp and Wp" = m · Wp, where n =1.1 to 1.5 and m= 1 to 3, and the magnetization pitches are λ, λ' and λ0 (where, λ< λ' << λ0) corresponding to Wp, Wp' and Wp" respectively. In this arrangement, the magnetic field intensity Hy in the reference position part 5a shown by a solid line in (B) of Fig. 7 is higher compared with a case where the magnetic poles are provided with the same magnetization width shown by a broken line in (B) of Fig. 7. As a result, the magnetic field intensity Hy in the reference position part 5a is maintained within the saturated magnetic field region, and consequently the output voltage Vm from the MR elements 7A and 7B is maintained at the saturation voltage V0. Accordingly the output voltage from the rotational angle sensor 6 can be stabilized at "0" value without fail. As aforementioned, the third embodiment can have the same effects and advantages as those of the first embodiment by maintaining the magnetic field intensity Hy in the reference position part 5a within the saturated magnetic field region. The first, second and third embodiments according to the present invention are described in the above on a case where ferromagnetic MR elements utilizing the circumferential element of the magnet as a magnetic signal are used. Instead of the ferromagnetic MR elements, however, Hall elements or any other magnetoelectric transducers having the saturation characteristics of the signal output can be used. The same effects can also be obtained by composing the sensor with the radial element of the magnet as a signal element. The magnet may be embodied in other forms as described below to obtain the magnetic field intensity Hy, which is higher than the saturated magnetic field intensity Hsat, in the reference position part 5a. Figs. 8 to 11 illustrate magnets in the fourth to seventh embodiments of the present invention, which are magnetized in accordance with the second embodiment. (Fourth Embodiment) As illustrated in Fig. 8, an annular isotropic ferrite type magnet 4 with a residual magnetic flux density Br of 0,1T to 0,4T (1000 to 4000G) is adhered to the outer periphery of the magnet holding part 3. The magnetized surface 5 of the magnet 4 is alternately provided with magnetic poles at the magnetization pitch λ . The magnet 4 includes the reference position part 5a alternately provided with magnetic poles with the magnetization pitch λ0 (λ0 = n·λ, where n = 2 to 4). The magnetism of the magnetic poles at the ends of the reference position part 5a is strengthened than that of any other magnetic poles by fitting a rare-earth type magnet with the residual magnetic flux density Br of 0,5T to 1,2T (5000 to 12000G) into the reference position part 5a as shown by N+ and S+ in Fig. 8. (Fifth Embodiment) As illustrated in Fig. 9, an inwardly and radially recessed part 33 is formed and magnetized at the magnetization pitch λ0 (λ= n·λ, where n = 2 to 4) in part of the outer periphery of the magnet holding part 3, and an annular isotropic ferrite type magnet 4 is adhered to the outer periphery of the magnet holding part 3. The reference position part 5a is provided in correspondence to the recessed part 33. In the reference position part 5a, the magnet 4 is made thicker in the radial direction than the other portions thereof to increase the permeance coefficient of the magnet 4 and strengthen the magnetism of the magnetic poles at the ends of the reference position part 5a as shown by N+ and S+ in Fig. 9. (Sixth Embodiment) As illustrated in Fig. 10, the recessed part 33 is formed and magnetized with the magnetization pitch λ0 (λ0 = n·λ, where n = 2 to 4) in part of the outer periphery of the magnet holding part 3, and a back yoke 34 composed of a magnetic material is adhered to the recessed part 33. An annular isotropic ferrite type magnet 4 is adhered to the outer periphery of the magnet holding part 3, and the reference position part 5a is provided in correspondence to the back yoke 34. In this arrangement, the magnetism of the magnetic poles at the ends of the reference position part 5a can be made stronger than that of any other magnetic poles as shown by N+ and S+ in Fig. 10. (Seventh Embodiment) As illustrated in Fig. 11, an annular back yoke composed of a magnetic material is adhered to the outer periphery of the magnet holding part 3, and an annular isotropic ferrite type magnet 4 is adhered to the outer periphery of the back yoke 35. The width th of the back yoke 35 in the radial direction is made wider than "λ/2." The magnetized surface 5 includes the reference position part 5a magnetized at the magnetization pitch λ0 (λ0 = n·λ, where n = 2 to 4). The back yoke 35 is arranged so that the effects thereof can be given only to the ends of the reference position part 5a, the magnetism of the magnetic poles at the ends of the reference position part 5a can be made stronger than that of any other magnetic poles as shown by N+ and S+ in Fig. 11. Although the fourth to seventh embodiments are described referring to an isotropic ferrite type magnet, a radial anisotropic ferrite type magnet and a rare-earth type magnet can also be applied thereto instead. Figs. 12 to 15 illustrate the eighth to eleventh embodiments, which are other versions of magnetization in accordance with the third embodiment of the present invention. (Eighth Embodiment) As illustrated in Fig. 12, an annular radial anisotropic ferrite type magnet 4 is adhered to the outer periphery of the magnet holding part 3. The magnet 4 includes the reference position part 5a left with the non-magnetized part with the width WP". The magnet 4 is alternately provided with magnetic poles of different directions with the magnetization width Wp. The magnetism of the magnetic poles provided in opposition to each other in the reference position part 5a is strengthened by being magnetized with the magnetization width Wp', where Wp' is larger than Wp. (Ninth Embodiment) As illustrated in Fig. 13, a radially outwardly projected part 36 with the width Wp" is formed on part of the outer periphery of the magnet holding part 3, and an annular radian anisotropic magnet 4 is adhered to the other part of the outer periphery of the magnet holding part 3. The magnet 4 is alternately provided with magnetic poles of different directions with the magnetization width Wp. The projected part 36 of the magnet holding part 3 corresponds to the reference position part 5a, and magnetic poles are provided in opposition to each other in the reference position part 5a with the magnetization width Wp', where Wp' is larger than Wp. (Tenth Embodiment) As illustrated in Fig. 14, an annular radial anisotropic magnet 4 is adhered to the outer periphery of the magnet holding part 3. The magnet 4 is alternately provided with magnetic poles of different directions and with the magnetization width Wp. The reference position part 5a is provided with magnetic poles in opposition to each other with the magnetization width Wp', where Wp' is larger than Wp. Furthermore, the reference position part 5a is alternately provided with a plurality of magnetic poles of different directions with the minute magnetization width Wh. As the narrower the magnetization width is, the lower the magnetic field intensity Hy is, when the magnetization width is minute and the magnetization direction is alternating as arranged in the above, the magnetic field intensity Hy in the reference position part 5a can be made extremely low. As a result, the magnetic field intensity Hy in the reference position part 5a can be made as if it was a non-magnetized part. (Eleventh Embodiment) As illustrated in Fig. 15, an annular radial anisotropic magnet 4 is adhered to the outer periphery of the magnet holding part 3. The magnet 4 is alternately provided with magnetic poles of different directions with the magnetization width Wp. The reference position part 5a is provided with magnetic poles in opposition to each other with the magnetization width Wp', where Wp' is larger than Wp. Furthermore, the reference position part 5a is alternately provided with a plurality of magnetic poles of different directions with the fine magnetization width Wh. Moreover, in this eleventh embodiment, the magnetic field intensity Hy in the reference position part 5a can be lowered than that of the aforementioned tenth embodiment of Fig. 14 by omitting the magnetized part with the minute width from the reference position part 5a in the radial direction. The invention having been described can improve the reference position detecting precision by stabilizing the output signal from the magnetoelectric transducers in the reference position part. This present invention may be applied for detecting a linearly moving position. The position detector therefore provides improved reference position detecting precision by stabilizing the output signal from magnetoelectric transducers in a reference position part. A rotor 2 has a magnetized surface 5 alternately provided with different magnetic poles and a reference positional part 5a in which magnetic poles provided on the magnetized surface 5 are partly omitted. A rotational angle sensor 6 has MR elements 7A and 7B laid out thereon in opposition to the magnetized surface 5 of the rotor 2 to output the voltage signal VS according to the magnetic field intensity in the magnetized surface 5 due to the rotation of the rotor 2 . A waveform processing circuit 11 and a pulse signal processing signal 12 shape the waveform of the voltage signal VS from the rotational angle sensor 6 , and also generate the rotational angle pulse signal Ne and a reference position pulse signal G according to the waveform reforming signal. The reference positional part 5a is arranged so that the magnetic field intensity therein can be within the saturation magnetic field region of the Mr elements 7A and 7B .

Claims: A position detector comprising: a moveable body (1, 2, 3) having a surface (5) magnetized in opposite polarities alternately at a first predetermined magnetization pitch (λ) on a surface thereof except at a reference position part (5a) which has a second magnetization pitch (λ0) larger than said first magnetization pitch, and a magnetoelectric transducer (6) positioned to face said moveable body (1, 2, 3) for generating a voltage signal (Vs) corresponding to an intensity of said magnetized surface (5), said transducer element (6) having an output saturation characteristics in a region where said magnetic field intensity is above a predetermined intensity (Hsat), characterized in that said magnetic field intensity caused by said reference position part (5a) in said magnetoelectric transducer (6) is above said predetermined intensity, when the reference position part (5a) is opposite said magnetoelectric transducer (6). A position detector according to claim 1, characterized in that said moveable body (1, 2, 3) comprises a rotor (2) having said magnetized surface (5) in the form of a circumferential surface and wherein the intensity of magnetized poles (N, S) provided just beside said reference position part (5a) is set to be higher than that of the other of said magnetic poles (N, S). A position detector according to claim 2, characterized in that said rotor (2) is so magnetized as to have a pair of magnetic poles (N, S) in a circumferential direction. A position detector according to claim 3, characterized in that said circumferential surface just beside said reference position part (5a) is made of a material which is different from other portion of said circumferential surface magnetized alternately. A position detector according to claim 3, characterized in that said circumferential portion just beside said reference position part (5a) is radially enlarged than the other portion of said circumferential portion. A position detector according to claim 1, characterized in that said position detector further comprises a rotor (2) having a circumferential surface magnetized as to havea pair of magnetic poles (N, S) arranged radially and the width of magnetization of magnetic poles (N, S) just beside said reference position part (5a) is set wider than that of the other magnetic poles (N, S). A position detector according to claim 6, characterized in that said circumferential portion between the magnetic poles (N, S) just beside said reference position part (5a) is non-magnetized. A position detector according to claim 6, characterized in that said circumferential position between the magnetic poles (N, S) just beside said reference position part (5a) is magnetized less than that of the other magnetic poles (N, S). A position detector according to claim 1, characterized in that said magnetoelectric transducer (6) includes a ferromagnetic reluctance element (7A, 7B), wherein said magnetic field intensity at said reference position part (5a) is so set as to be in a region where said ferromagnetic reluctance element (7A, 7B) has no hysteresis.

Inventors: AOYAMA SEIKI, KAJIMOTO KAZUO, MAKINO TOMOATSU, MAKINO YASUAKI, AOYAMA, SEIKI, KAJIMOTO, KAZUO, MAKINO, TOMOATSU, MAKINO, YASUAKI
Applicants: NIPPON DENSO CO, NIPPONDENSO CO., LTD.
Application year: 1994
Main classification: G01D5
IPC codes: F02D41, G01D5